Bump postcss and gulp-autoprefixer

Bumps [postcss](https://github.com/postcss/postcss) to 8.4.31 and updates ancestor dependency [gulp-autoprefixer](https://github.com/sindresorhus/gulp-autoprefixer). These dependencies need to be updated together.


Updates `postcss` from 7.0.39 to 8.4.31
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@7.0.39...8.4.31)

Updates `gulp-autoprefixer` from 7.0.1 to 8.0.0
- [Release notes](https://github.com/sindresorhus/gulp-autoprefixer/releases)
- [Commits](sindresorhus/gulp-autoprefixer@v7.0.1...v8.0.0)

---
updated-dependencies:
- dependency-name: postcss
  dependency-type: indirect
- dependency-name: gulp-autoprefixer
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <[email protected]>

package.json

@@ -16,7 +16,7 @@
     "del": "^5.1.0",
     "gemfile": "1.1.0",
     "gulp": "^4.0.2",
-    "gulp-autoprefixer": "^7.0.1",
+    "gulp-autoprefixer": "^8.0.0",
     "gulp-clean-css": "^4.3.0",
     "gulp-concat": "^2.6.1",
     "gulp-imagemin": "^6.2.0",